This invention relates to valves and the construction thereof.
Conventional gate valves, swing-check valves, and globe valves have been utilized in a myriad of applications. Problems have been encountered in certain operating conditions involving high pressure ranges, highly corrosive fluids, high temperature ranges, and a high number of cycles. In these extreme operating conditions, connections between internal valve parts fail, requiring repair or replacement of the valve. Generally, the valves tend to fail in areas subject to sliding contact. The failure of the connections is intensified by the required use of different materials. Specifically, the valve body which acts as a pressure vessel must be a strong, tough, and somewhat ductile material with a high modulus of elasticity while the guide channel, valve seats, and facing ring which serve as wearing surfaces must be of a harder less ductile material. These different materials typically have diverse thermal expansion coefficients resulting in diverse amounts and rates of expansion and contractions between the parts due to temperature changes. If the parts are restrained by welding, the welds will be subject to high thermal stresses, eventually resulting in weld failure.
It is well known that guide channels which guide a wedge in and out of closed position in a valve can be attached to allow for thermal expansion by forming an aperture with a counterbore in the guide channel, inserting a pin having a rim through the aperture, and welding the pin to the body of the valve. The rim is sized to fit into the counterbore, engage the guide channel, and hold the guide channel against the body. The guide channel is allowed to expand and contract with temperature because the pin and the rim are of such a size that there is a gap between the outer circumference of the pin and the rim and the inner circumference of the aperture and the counterbore. However, the weld is not immune from thermal stress, and the weld is further subject to attack in a highly corrosive and high cycle environment. If a weld fails, the guide channel can no longer serve its function of properly positioning the wedge in the closed position.
It is also conventional to weld a facing ring, which seals against the valve seat, to a wedge or disc, hereafter referred to as the stop, of a valve. Though the stop is not a pressure vessel, it must still retain pressure. Thus, the thermal expansion problem also exists between the stop and facing ring. Again, the weld is subject to thermal stresses from the differential thermal expansion of the stop and the facing rings and from weakening by corrosive fluids. A facing ring weld failure prevents the facing ring from sealing against the valve seat and, if the failure is extensive, can result in an inability to close the valve and damage to the internal valve parts.
A valve seat is typically attached to the valve body with a circumferential weld which is subject to the same thermal stresses and corrosion as the welds of the guide channels but without the benefit of the pin and aperture connection discussed above. Failure of the connection between the valve seat and the body results in an inability to close the valve and possible damage to the valve. The art has proposed a valve seat which is threaded into the valve body, but forming the threads in the valve seat and in the valve body greatly increases the cost of the valve.
These problems require routine maintenance to replace parts or even entire valves. Thus, reduction in the required maintenance and replacement of parts in gate, swing-check, and globe valves is desirable to reduce the maintenance cost and increase the useful life of the valves. Such an increase in the life of the valve translates directly into a reduction of cost.